Liquid crystal display devices are electronic devices for converting electrical signals generated from various devices into visual information by using variations in the transmissivity of liquid crystals caused by a voltage applied thereto. Owing to merits such as small size, light weight, and low power consumption, liquid crystal display devices have been considered substitutes for cathode ray tubes (CRTs) and are now used in most information processing apparatuses requiring display devices.
In such a liquid crystal display device, a voltage is applied to liquid crystals to change the molecular arrangement thereof and thus change optical characteristics of the liquid crystals such as birefringence, optical rotatory power, dichroism, and light scattering. That is, liquid crystal display devices modulate light using liquid crystals. Since liquid crystal display devices do not produce light themselves, additional light sources are necessary to illuminate liquid crystal device screens. Such illumination light sources are generally called backlight units.
Backlight units may be classified as edge-type backlight units and direct-type backlight units, according to the position of a light emitting lamp. Edge-type backlight units include a lamp at a side of a light guide panel that guides light emitted from the lamp. Edge-type backlight units are used in small liquid crystal display devices such as desktop computer monitors and laptop computers and are advantageous in terms of evenness of lighting, durability, and size reduction. On the other hand, direct-type backlight units are developed for application in display devices having 20-inch or larger screens. Such a direct-type backlight unit includes lamps arrayed under a liquid crystal panel to directly illuminate the entirety of the liquid crystal panel.
Linear light sources such as cold cathode fluorescent lamps (CCFLs) were previously widely used as lamps for backlight units. However, light emitting diodes (LEDs) are now widely used as lamps for backlight units due to merits thereof, such as good color reproductivity, environmental-friendliness, slimness and lightness, and low power consumption.
Such a backlight unit may include a plurality of optical films for diffusing or collecting light emitted from a light source, improving brightness, or reducing uneven lighting known as “lamp mura.” FIG. 1 is a view illustrating an exemplary direct type backlight unit of the related art. Referring to FIG. 1, a plurality of light sources 2 are arranged on a reflection plate 1, and a diffusion plate 3 is disposed above the light sources 2 to make the brightness of an entire screen uniform and support optical films. At least one diffusion film 4 and at least one condensing film 5 may be used for condensing light diffused by the diffusion plate 3 according to an effective visible range and improving the uniformity of screen brightness.
Examples of the condensing film include a prism film, a lenticular film, and a micro-lens array (MLA) sheet. The MLA sheet has a light condensing function as well as a light diffusing function. However, if micro lenses are periodically arranged on the MLA sheet as shown in FIG. 2, when the MLA sheet is superposed on another optical sheet having a periodic arrangement or pattern or an LCD panel having periodic pixels, a moiré pattern may occur due to geometric interference.
The term “moiré pattern” refers to an interference pattern generating when two or more patterns are superimposed on one another. For example, if two optical films having similar light and shade patterns are superimposed, a new light and shade pattern results, known as a “moiré pattern.”
Since such an unnecessary pattern may be included in an image displayed on an LCD due to a moiré pattern, it is necessary to prevent or remove such a moiré pattern by a method such as removing periodicity from patterns that are superimposed. In this case, however, brightness can be remarkably reduced.
The brightness and viewing angles of image display devices are important characteristics and are known to be determined by the characteristics of backlight unit optical sheets. In recent years, hemispheric micro-lens array sheets have been widely used. However, such hemispheric micro-lens array sheets have limitations in improving brightness and result in decreases in brightness when increasing viewing angles.
Therefore, there is a need to develop a micro-lens array sheet comprising micro-lens arrangement having a moiré pattern occurrence while minimizing decreases in brightness.